1. Field of the Invention
This invention relates to a pneumatic radial tire using polyester cords in a carcass and more particularly to a pneumatic radial tire for passenger car simultaneously improving a steering stability and a ride comfort against vibration without sacrificing a durability.
2. Description of Related Art
In general, the steering stability and ride comfort against vibration are considered to be most important performances as a property required in the pneumatic radial tire for passenger car. Recently, the running performances of vehicles mainly running on paved road, particularly passenger cars are improved every year and used under good maintenance, and also the road system is more expanded, so that it is demanded to more improve the running performances, particularly steering stability of a tire used for such a vehicle. Furthermore, as the ride comfort of the passenger car is improved with the servicing of road surface, it is strongly demanded to improve ride comfort against vibration when the tire rides on projections such as asphalt joints in a bridge or an expressway and so on.
For this end, various examinations and developments are made for satisfying these performances. Among them, cords of polyethylene terephthalate fiber (PET) having a relatively high modulus at an initial tension as a polyester cord are usually and widely used in a carcass of the radial tire for passenger car. In this case, however, a stiffness of a sidewall portion in the tire becomes high, which badly affects the ride comfort against vibration. Furthermore, the carcass containing such PET cords therein can not sufficiently respond to the demand for the steering stability and ride comfort accompanied with the improvement of the performances of the vehicle.
In order to improve the steering stability and ride comfort of the tire, it is advantageous to make the carcass tough or to enhance the modulus of the cord constituting the carcass. Therefore, it is considered that polyethylene naphthalate fiber cord (PEN cord) having a higher modulus of elasticity is used instead of the existing PET cord among the polyester fiber cords as a cord constituting the carcass. However, the PEN cord is poor in the fatigue resistance because the modulus of elasticity is higher than that of the conventional PET cord, so that when the PEN cord is used in the carcass, it is feared that the durability of the tire is not guaranteed.
It is, therefore, an object of the invention to provide a pneumatic radial tire simultaneously improving the steering stability and the ride comfort against vibration by using polyester cords having specified properties in the carcass of the tire without sacrificing the durability.
The inventor has made various studies with respect to the steering stability and ride comfort against vibration of the tire when the polyester cords having specified properties are used in the carcass of the radial tire and found that the steering stability and the ride comfort against vibration can simultaneously be improved by controlling dynamic stiffness of the tire. That is, it has been confirmed that tension applied to the carcass is high in the sidewall portion and low in the tread portion owing to the presence of the belt even when the internal pressure or the structure of the tire is same. As a result, the invention has been accomplished by masking the dynamic modulus of the polyester cord used in a carcass ply lower at a low tension and higher at a high tension as compared with that of the conventional PET cord. Furthermore, the inventor has made studies with respect to the fatigue resistance and the tire durability and found the following facts:
(a) In general, the organic fiber cord hardly causes fatigue at tensile strain but causes fatigue at compression strain;
(b) The compression strain is applied to a turnup portion of the carcass ply wound around a bead core from an inside of the tire toward an outside thereof;
(c) The compression strain in the turnup portion becomes smaller as the bending of the bead portion becomes small when a load is applied to the tire;
(d) In case of a tire having a small aspect ratio, the stiffness is high and the bending of the bead portion is small, so that strain applied to the turnup portion is small, which are advantageous in the fatigue resistance of the cord in the carcass; and
(e) As the stiffness of the bead portion becomes high, the deformation of the sidewall portion is gentle and strain applied to the cord in the carcass decreases.
The invention is based on the above knowledge.
According to the invention, there is the provision of in a pneumatic radial tire comprising a pair of bead portions, a pair of sidewall portions, a tread portion, a radial carcass toroidally extending between a pair of bead cores embedded in the bead portions and comprised of at least one rubberized ply containing organic fiber cords therein, and a belt superimposed about a crown portion of the carcass and comprised of at least two belt layers containing steel cords arranged at a given cord angle with respect to an equatorial plane of the tire, an improvement wherein the organic fiber cord used in the carcass is a double-twisted polyester cord of 1000-10000 dtex having a dynamic modulus at a tension of 200 g of 4xc3x971010-6xc3x971010 dyn/cm2 and a dynamic modulus at a tension of 1500 g of 6xc3x971010-9xc3x971010 dyn/cm2.
In a preferable embodiment of the invention, the polyester cord is a polyethylene naphthalate fiber cord.
In another preferable embodiment of the invention, the tire has an aspect ratio of not more than 65%.
In the other preferable embodiment of the invention, the bead core is formed by helically winding a steel wire having a diameter of 1.2-1.8 mm 7-30 times.